The present invention relates to acoustic resonators, and more particularly, to resonators that may be used as filters for electronic circuits.
The need to reduce the cost and size of electronic equipment has led to a continuing need for ever smaller filter elements. Consumer electronics such as cellular telephones and miniature radios place severe limitations on both the size and cost of the components contained therein. Many such devices utilize filters that must be tuned to precise frequencies. Hence, there has been a continuing effort to provide inexpensive, compact filter units.
One class of filters that has the potential for meeting these needs is constructed from thin film bulk acoustic resonators (FBARS). These devices use bulk longitudinal acoustic waves in thin film piezoelectric (PZ) material. In one simple configuration, a layer of PZ material is sandwiched between two metal electrodes.
The sandwich structure is preferably suspended in air by a support structure. When electric field is applied between the metal electrodes, the PZ material converts some of the electrical energy into mechanical energy in the form of mechanical waves. The mechanical waves propagate in the same direction as the electric field and reflect off of the electrode/air interface.
At a resonant frequency, the device appears to be an electronic resonator. When two or more resonators (with different resonant frequencies) are electrically connected together, this ensemble acts as a filter. The resonant frequency is the frequency for which the half wavelength of the mechanical waves propagating in the device is equal to the total thickness of the device for a given phase velocity of the mechanical wave in the material. Since the velocity of the mechanical wave is four orders of magnitude smaller than the velocity of light, the resulting resonator can be quite compact.
In designing and building miniature filters for microwave frequency usage, it is often necessary to provide multiple interconnected resonators (for example, FBARS) fabricated on a die. FIG. 1 is a schematic diagram showing a portion 10 of a filter circuit. For convenience, the illustrated portion is referred to herein as the “filter circuit” 10. The filter circuit 10 includes a plurality of interconnected resonators. Referring to FIG. 1, some of the illustrated resonators are connected in series and are referred to as series resonators 12, 14, and 16 while other illustrated resonators are connected in parallel and are referred to as shunt resonators 22, 24, 26, and 28. The filter circuit 10 connects to external circuits (not illustrated) via connection points 11, 13, 15, 17, 19, and 21.
FIG. 2 shows a top view of a die 20 illustrating topology of the resonators of the filter circuit 10 FIG. 1 as they are typically implemented on the die 20. In FIGS. 1 and 2, corresponding resonators are illustrated with same reference numerals. Connection points of FIG. 1 are illustrated as connection pads in FIG. 2 and corresponding connection points and connection pads are illustrated with same reference numerals.
As illustrated, the die 20 requires a die area (defined by the first and second dimensional extents illustrated as X-axis extent 23 and Y-axis extent 25) to implement the resonators. Die area is a scarce and expensive resource in many electronic devices, for example, wireless communication devices such as cellular telephones. It is desirable to be able to implement the filter circuit 10 on a smaller die allowing for manufacture of smaller and less costly devices.